In recent years, in an attempt to further increase speed and capacity of wireless communication in a wireless communication system, such as a mobile phone system (cellular system), discussions are held on wireless communication technology for the next generation. For example, in the 3GPP (3rd generation partnership project), which is a standards organization, a communication standard called as LTE (long term evolution) and a communication standard called as LTE-A (LTE-advanced) based on the wireless communication technique of LTE are suggested.
The latest communication standard completed in the 3GPP is Release 10 compatible with LTE-A, in which Releases 8 and 9 compatible with LTE are greatly functionally enhanced. Currently, discussions are held for completion of Release 11, in which Release 10 is further enhanced. In the descriptions below, “LTE” includes, in addition to LTE and LTE-A, other wireless communication systems that are made by enhancing LTE unless otherwise noted.
In Release 11 of the 3GPP, coordinated multiple point (CoMP) is one of the techniques that are discussed actively in particular. Simply put, CoMP is a technique to coordinate sending and receiving to a wireless terminal (UE: user equipment) between different wireless base stations (eNB: evolved node B). In the following descriptions, a wireless link in a direction from a wireless terminal to a wireless base station is called as an uplink (UL) and a wireless link in a direction from a wireless base station to a wireless terminal is called as a downlink (DL).
There are several forms of CoMP, and a scenario is known in which a wireless terminal carries out communication with different wireless base stations for UL and DL. In a regular case, a wireless terminal carries out communication with an identical wireless base station for UL and DL. That is, it is common that a wireless terminal carries out communication with a connected wireless base station (serving cell). As an example, a wireless terminal receives UL scheduling information from a connected wireless base station on a DL and sends data to the connected wireless base station on a UL based on the received UL scheduling information. As another example, a wireless terminal receives data from a connected wireless base station on a DL and sends a response signal (ACK signal or NACK signal) to the received data to the connected wireless base station on a UL.
However, there is sometimes another wireless base station whose communication quality of UL for the wireless terminal is higher than the connected wireless base station. Even when certain communication quality of DL (receiving quality, propagation delay, and the like) is secured, in such a case that a wireless terminal is located at a cell edge, the communication quality of UL may not be good. When there is another wireless base station whose communication quality of DL is higher than the connected wireless base station, the connected wireless base station of the wireless terminal is switched by handover, so that this is out of the problem.
Such problem easily occurs as well in a so-called heterogeneous network, for example, where macrocells, which are regular base stations, and picocells and the like (also microcells, femtocells, and the like), which are small base stations, are mixed. In a heterogeneous network, a picocell is desired to suppress the DL cell size (suppress the sending power) in order to inhibit interference in a macrocell. That is, a macrocell and a picocell greatly differ in DL cell size (sending power). Therefore, a situation may occur that, for a wireless terminal located at a cell edge of the connected macrocell, the DL quality is better with a macrocell while the UL quality is better with a picocell.
In the 3GPP, in view of such problems, a scenario is under review in which a wireless terminal carries out communication with different wireless base stations for UL and DL as described above. In this scenario, when there is another wireless base station whose communication quality of UL for a wireless terminal is higher than the connected wireless base station, the wireless terminal carries out UL communication with the base station different from the connected wireless base station. As an example, it is possible that the wireless terminal receives UL scheduling information from the connected wireless base station on a DL and sends data to a base station different from the connected wireless base station on a UL based on the received UL scheduling information. As another example, it is possible that a wireless terminal receives data from the connected wireless base station on a DL and sends a response signal to the received data to a base station different from the connected wireless base station on a UL. In such a manner, it becomes possible to secure the UL wireless communication quality to a wireless terminal that has poor UL wireless quality with the connected wireless base station. Then, as a result, an effect of improving transmission efficiency of the entire system is expected.
Non Patent Literatures 3GPP TS36.211 V10.4.0 (2011 December), 3GPP TS36.213 V10.4.0 (2011 December), 3GPP TR36.814 V9.0.0 (2010 March), 3GPP TR36.819 V11.0.0 (2011 September), 3GPP R1-114324 “On Reference Signal Enhancements for UL CoMP” (2011 November) are examples of the related art.